In recent years a substantial amount of research and engineering effort has been directed to the storage of electrical energy so that it would be available quickly and efficiently when needed, such as during high energy demand periods in the summer for air conditioning and in the winter for heating. It is also desirable to store electrical energy produced during the nighttime when consumption is low so that it is available for daytime use for peak shaving when demand is much greater, thereby permitting a power plant to run at a more uniform rate.
Electrical energy storage also may be used when it is desirable to generate power at a lower rate than at which it will be consumed, store the generated power in the form of electrical energy and subsequently release the stored energy to meet high rate consumption demands.
One form of electrical energy storage which has been studied extensively is the superconducting magnetic energy storage (SMES) system which is intended to operate at very low temperatures, i.e. cryogenic temperatures. One such system comprises a circular coil surrounded by a coil containment vessel containing liquefied helium at a temperature of 1.8.degree. K. The liquefied helium cools the coil, generally aluminum and niobium-titanium, to make it superconducting by lowering electrical resistance. The coil containment vessel in turn is surrounded by a vacuum vessel, the main function of which is to minimize heat loads on the cryogenic system. A shroud between the coil containment vessel and the vacuum vessel, but surrounding the coil containment vessel, is generally also included to further prevent heat transfer. This is achieved by cooling the shroud with liquefied nitrogen. The entire apparatus as described is to be installed in a large circular trench or tunnel having inner and outer circumferential walls constructed to accept the compressive loads applied during operation of the SMES apparatus.
After a SMES apparatus is constructed and is ready to be put in use the vacuum vessel is evacuated to a suitable vacuum. This causes the vacuum vessel walls to move towards each other and also radially inwardly. The shroud is then cooled following which the coil is cooled down by filling the coil containment vessel with liquefied helium. This cooling causes the coil and coil containment vessel to contract and to move radially inwardly. After the coil is cooled to its operating temperature, the superconducting coil is charged with electricity. The charged coil produces a large radial outward magnetic load which is partially offset by the vacuum and cooldown loads. In addition to the described loads, long term creep of the surrounding foundation will occur.
The charged coil also produces a large vertical comprehensive load within the coil itself which actually reduces the height of the coil. Thus, all of these loads and movements must be accommodated while maintaining the structural and operating integrity of the SMES apparatus. This requires a coil containment vessel able to withstand a fraction of the loads and be able to transfer the remainder to an external support system. The coil containment vessel must also be able to withstand an internal pressure from the liquid helium used to cool the coil.
Coil containment vessels are generally quite large. The radius of the vessel can be one hundred to six hundred or more feet and it can be from ten to one hundred or more feet in height. As a result, construction of the vessel is difficult as the component parts are large and must be assembled below grade or in a tunnel.
From the above discussion it is believed clear that a flexible and mass-producible coil containment vessel would be useful.